1. Field of the Invention
The present invention relates to image-forming apparatuses in which image-bearing members and transfer members that are in contact with the image-bearing members are driven to rotate.
2. Description of the Related Art
To form good images with high accuracy in electrophotographic image-forming apparatuses, it is desired that photoconductive members and transfer members in contact with the photoconductive members be driven by drive units with high rotational accuracy. This is because nonuniformity in the driving operation of the drive units may lead to image failure including color misregistration, banding, and blank spots.
Typically, in a color-image-forming apparatus, color misregistration occurs because of shifts in the relative positions of images formed in different colors. One of the causes for such shifts in the relative positions of images is nonuniformity in the operation of driving photoconductive members and transfer members. Banding is variation in density periodically occurring in an image. Banding occurs because of periodical changes in the circumferential speeds of each photoconductive member and the corresponding transfer member during image formation. Blank spots occur because of positional shifts of toner during transfer from each photoconductive member to the corresponding transfer member performed at a transfer nip produced therebetween. The positional shifts of toner at the transfer nip occur because of the relative speed difference between the photoconductive member and the transfer member.
It is known that, in a configuration where the driving force of a motor is transmitted to a photoconductive member or a transfer member through reduction gears, the nonuniformity in the operation of driving the photoconductive member or the transfer member is reduced by detecting the angle of rotation of the photoconductive member or the transfer member, not the angle of rotation of the motor, and feeding the result of detection back to the motor. Thus, the low-frequency component of the nonuniformity in the driving operation is reduced, whereby color misregistration can be suppressed. Such a technique, however, is not effective in reducing the high-frequency component of the nonuniformity in the driving operation caused by the transmission of the driving force through the gears, and it is still difficult to suppress banding and the occurrence of blank spots.
There is a known technique in which a photoconductive member is driven by an oscillatory-wave motor (also known as a vibration-type motor or vibration wave motor) that does not require speed reduction with gears but produces a relatively large torque (as disclosed in Japanese Patent Laid-Open No. 10-186952, for example). Oscillatory-wave motors produce driving forces by exciting oscillatory bodies to generate oscillatory waves and perform relative friction driving of contacting bodies that are in contact with the oscillatory bodies (see Japanese Patent Laid-Open No. 60-176470, for example).
In Japanese Patent Laid-Open No. 10-186952, the photoconductive member is directly driven by an oscillatory-wave motor and the transfer member is driven by a pulse motor with gears interposed therebetween. The circumferential speed of the transfer member is controlled in accordance with the circumferential speed of the photoconductive member. Thus, the photoconductive member and the transfer member can be driven without nonuniformity in the driving operation. In image-forming apparatuses, however, the torque for driving the transfer member is larger than the torque for driving the photoconductive member. To drive such a transfer member by a motor with no gears interposed therebetween, a large motor is required. This is disadvantageous in terms of manufacturing cost and space. Nevertheless, if the photoconductive member is directly driven by an oscillatory-wave motor and the transfer member is driven by a pulse motor or a direct-current (DC) motor with gears interposed therebetween, the high-frequency component of the nonuniformity in the driving operation produced by the transmission of the driving force with the gears cannot be reduced effectively.